In recent years, a renewable energy power generation device for generating electric power from wind power, solar photovoltaic energy, geothermal energy, or the like has attracted attention and been put to practical use. The renewable energy power generation device is a clean power generation device that does not use any limited resource such as oil but uses an energy resource present in unlimited quantity in nature, and can suppress carbon dioxide emissions. Hence, the renewable energy power generation device has been increasingly introduced by companies, governments, and the like, for prevention of global warming.
However, since energy coming from nature varies from moment to moment, output fluctuations are unavoidable in the renewable energy power generation device. This poses an obstacle to widespread use of the renewable energy power generation device. To remove this obstacle, in the case of employing the renewable energy power generation device, it is preferable to build an interconnected (power generation) system that combines the renewable energy power generation device with an electric power storage-compensation device having a plurality of sodium-sulfur batteries (secondary batteries) as a main component.
A sodium-sulfur battery has features such as a high energy density, an ability to produce high output in a short time, and an excellent high-speed response. Accordingly, when a bidirectional converter for controlling charge and discharge is added, the sodium-sulfur battery offers an advantage that output fluctuations of the renewable energy power generation device which can occur on the order of several hundred milliseconds to several seconds can be compensated. Therefore, the interconnected system in which the renewable energy power generation device is combined with the electric power storage-compensation device having the plurality of sodium-sulfur batteries as the component is a desirable power generation system.